Pipe for carrying fluids, particularly hydrocarbons

ABSTRACT

The invention describes a pipe for carrying fluid, including thermally insulated steel pipe sections of a type including an outer tubular shell, an inner cylindrical shell of smaller diameter, in the inner shell being sealingly welded to the outer shell at each of the ends of the pipe sections. It is characterized in that at each joint, the ends of the two inner shells are welded to each other, and a sleeve links to the ends of the outer shells so that when traction, compression or bending forces are exerted on the pipe, the said sleeve transmits these forces to the outer shells.

The present invention relates to a pipe for carrying fluids,particularly hydrocarbons, made by assembling heat-insulated metaltubes.

It is known that in a number of cases, crude oil is extracted from theground at a temperature which is several tens of degrees greater thanthe ambient temperature and that, when this crude oil containsparaffinic products, its cooling to ambient temperature (or even totemperatures greater than the ambient temperature) causes for exampleprecipitation of these products which progressively leads to blockage ofthe pipe.

It has already been attempted to overcome these difficulties bysurrounding the pipes with thick insulators based on expanded materialswhich must themselves be protected by a layer of rigid materials, so asto withstand the various stresses to which the pipe is subjected bothwhen it is fitted and when it is used.

These insulators have the drawback of being bulky, and of considerablyincreasing the external dimensions of the pipe. They also greatlyincrease its buoyancy, which often requires its ballasting to beincreased.

In the latter case, it is then necessary to weight the pipe by encasingit in concrete, which is almost always difficult to make adhere to theouter wall of the insulator.

These known pipes also have the drawback of corroding easily when theouter insulator becomes degraded or because of the thermal expansionswhich are different between the steel tube and the insulator.

It also been envisaged to produce insulating tube sections consisting ofan outer tubular element and an inner tubular element which are weldedat their ends and which contain between them an insulating element inorder to constitute pipes.

The use of such tube sections has proved unsatisfactory, in particularbecause of the difficulties which are encountered in joining the tubularelements together in order to constitute the pipes.

The object of the present invention is to obtain, in a simple andeconomical manner, a pipe produced from thermally insulated steel tubesections, but which include no outer insulator and which can be handled,fitted and used for producing submerged or overland pipes, exactly inthe same way as conventional steel tube sections which include noinsulator.

The subject of the present invention is a pipe for carrying fluids,particularly hydrocarbons, consisting of thermally insulated steel tubesections, of the type comprising an outer tubular steel shell, an innercylindrical steel shell of smaller diameter, the inner shell beingwelded in a leaktight manner to the outer shell at each of the ends ofthe tube sections, while leaving between the two shells a closed annularvolume, preferably containing a heat insulating material, which pipe ischaracterized in that, at each joint between two tube sections, the endsof the inner shells of the two pipe sections are welded to one another,while a sleeve joins the ends of the outer shells, such that whentraction, compression or bending forces are exerted on the pipe, thesaid sleeve communicates these forces to the outer shells whileimparting to the pipe a mechanical strength at each joint which is atleast equal to that of the tube sections.

According to a first embodiment of the invention, the sleeve is screwedonto each outer shell by a cylindrical thread, preferably with straightturn fronts, which includes between the male and female turns aclearance which is sufficient to compensate for alignment andpositioning defects in the two sections when they are joined by weldingtheir inner shells, and the space contained between the male turns andthe female turns is lined with a substance, for example a polymerizablesubstance, which transmits the compression or traction forces which areto be exerted between the sleeve and the outer shells.

In order to do this, the total surface areas of the turn flanks must bechosen to be sufficiently large to allow for the crushing strength ofthe substance which is introduced between the male turns of the outershells and the female turns of the sleeve.

This embodiment makes it possible, in a simple and economical manner, toconnect two pipeline sections by a single weld which joins the innershells in a leaktight manner, the outer shells being joined to eachother by the substance, which can most often be applied cold, which isplaced in the threads.

Moreover, the clearance between the turns of the sleeve and of the outershells is sufficient to compensate for any positioning defects which mayresult from the joining of the two sections by welding their innershells, either in the case of alignment defects or parallax of the axesof the two sections or alternatively shifts in pitch between the turnsof the two outer shells.

The substance introduced between the turns may for example be a resinsuch as an epoxy resin or an araldite which polymerizes not withcontraction, but preferably with expansion.

It is also possible to use polymerizable elastomers and products such ascoal pitches which are applied hot and which are allowed to cool.

In a second variant of this first embodiment, the forces are transmittedbetween the sleeve and the outer shells of the two sections by virtue ofthe fact that, when the pipeline bench, the male turns of the outershells of the two sections bear on the female turns of the sleeve. Inorder to allow bending of the inner shell at the joint which issufficient to ensure compensation for the clearance existing between theturns of the sleeve and those of the outer shells, the collars joiningthe inner and outer shells to the end of each section are welded ontothe inner shells at a sufficient distance from the ends of the innershells.

In this variant, a free length of inner shell is left at a joint whichmay for example be of the order of one meter.

This characteristic is also advantageous in the first variant of thisembodiment because it makes it possible to ensure mechanical continuityof the pipeline, even in the event that the substance injected onlypartially occupies the space between the turns of the sleeve and of theouter shells, and therefore has only an insufficient mechanicalstrength.

In a second embodiment of the invention, the sleeve is fitted at one ofits ends with an internal thread which is screwed onto a correspondingexternal thread made on the end of the outer shell of a first tubesection, such that the other end of the said sleeve is applied against astop integral with the outer tubular shell of a second neighbouring tubesection with a sufficient force for the pipe to have, at the jointsbetween the various sections, a mechanical strength at least equal tothat of the other parts of the pipe.

According to a preferred variant, the thread by which the sleeve isscrewed onto the outer tubular shell of the first tube section is acylindrical thread with bearing turn fronts substantially perpendicularto the axis of the pipe, so as to allow compensation for the slightparallaxes which may be produced when the inner tubular shells arejoined by welding and which lead to the axes of the various sections ofthe pipe being slightly offset, albeit remaining parallel to each other.

According to this embodiment, the sleeve is screwed so as to impart toit, during mounting, an axial compression of a value greater than themaximum extension that some of its generatrices may undergo when thepipe is fitted or used.

According to the invention, it is advantageous, after screwing, for thesleeve to be welded at at least one of its ends onto the outer shell ofone of the sections, so as to prevent it unscrewing accidentally.

The welding at each end of the sleeve makes it possible to isolate theannular volume contained between the sleeve and the inner shells, andthus to prevent introduction into this volume of fluids outside the pipewhich might cause corrosion.

In one variant of this second embodiment, the stop on which the threadedsleeve bears when it is screwed onto the other section is fitted with aspherical ring which makes it possible automatically to compensate forthe defects in alignment between the axis of the sleeve and that of thestop.

According to the two embodiments described hereinabove for preventingthe introduction of external fluids into the annular volume containedbetween the sleeve and the inner shells, this volume may be filled forexample with a bituminous substance or a cellular material such as apolyurethane foam, which has the additional advantage of increasing theinsulation of the pipe.

In a preferred implementation of the invention, the welding of the endsof the inner shell and of the outer shell of the same section is carriedout after having imparted to the inner shell an extension with respectto the outer shell which corresponds substantially to that which existswhen the inner shell is subjected, with respect to the outer shell, to atemperature difference which is approximately equal to half thetemperature difference which will exist when the pipe is in use andwhich will cause heating of its inner shell.

According to an advantageous implementation of this characteristic, theinner shell is preheated to approximately half the temperaturedifference which is to exist between the inner shell and the outer shellwhen the pipe is used, and the ends of the two shells are then connectedby welding them in this state.

The result of this is that, when a tube section according to theinvention is at a homogeneous temperature, the inner shell is in a stateof extension, whereas the outer shell is in a state of compression. Thissituation develops progressively when a hot fluid is made to flow-insidethe pipe, the outer shell then being progressively compressed, whereasthe inner shell enters progressively into traction [sic].

According to the invention, it is advantageous when the pipe is to beused for carrying a fluid at high temperature, to make the inner shellsand possibly the collars from a metal which has a low coefficient ofexpansion, such as for example the one known under the designationInvar.

It is thus possible to use the pipe according to the invention forcarrying steam at more than 100° C.

According to the invention, the inner and outer shells may have equalthicknesses, but it is in general advantageous for the inner shells ofthe sections which are to be welded together for making the pipesections to have a thickness greater than that of the outer shells whosecontinuity is ensured by the sleeve.

Thus, the inner shell may for example have a thickness approximately 3to 4 times greater than that of the outer shell.

According to a particular embodiment of the invention, the inner andouter shells are joined together by welding using collars whosethickness is substantially equal to the thickness of the thinnest shell.

In order to limit the magnitude of the thermal bridges which result fromtheir presence, these collars extend over a length which preferably liesbetween 3 and 5 times the distance which separates the outer face of theinner shell from the inner face of the outer shell.

In one particular embodiment of the invention, the collars which jointhe two shells at their ends have a cross section in the general shapeof an S or of a half sine wave.

In one variant, the collars have a cylindrical shape and their ends arewelded onto annular bosses integral with the ends of the inner and outershells which they join.

In another variant, the collars each have a cylindrical central partextended by two frustoconical parts welded directly to the outer tubularshell, and to the inner tubular shell.

According to the invention, the collars may advantageously be made froman alloy which has a low sensitivity to heat, such as that known underthe name Invar.

According to preferred embodiment of the invention, the closed annularvolume contained between the two steel shells is lined with a pluralityof thin sheets of an anti-thermal radiation insulator, the said sheetsbeing preferably separated by cavity structures which do not conductheat, which prevent them being applied against each other. Suchinsulating sheets may for example consist of aluminium sheets with athickness of a few hundredths of a millimeter.

According to another embodiment of the invention, the closed annularvolume may be filled with an expanded synthetic material such as forexample expanded polyurethane or polyethylene.

Considering the fact that tube sections which are usually available havelengths of approximately 12 meters, and that these tubes are subjected,especially when fitting the pipe, to strong bending forces, provision ismade according to a preferred embodiment of the invention, to locate atappropriate intervals, for example every meter or every two meters,spacer members which are placed between the inner shell and the outershell.

These spacer members must be made of a material which has sufficientmechanical strength but which is not a good conductor of heat, forexample from a synthetic material.

In order to limit the thermal bridges constituted by the said spacermembers, it is recommended according to the invention not to give them aconstant cross section over the entire perimeter of their winding. Thesemembers may for example be in the form of blocks which are of sufficientsize to prevent, during for example bending stresses, the inner shellfrom collapsing against the outer shell, the said blocks being joinedtogether by thinner sections of material, which thus limits heattransfer.

According to the invention, it is advantageous, when the tube section issubjected to no abnormal stress, for the spacer members to come intocontact with only one of the shells.

According to another preferred embodiment of the invention, theinsulating character of the pipe is substantially improved by creatingin the space contained between the inner shell and the outer shell ahigh vacuum which may for example be of the order of 10⁻⁵ to 10⁻⁸ bar.

This evacuation may be carried out by conventional techniques whichgenerally involve prior degassing of the metal surfaces by cleaning orby heating.

In one variant, the annular volume contained between the inner shell andthe outer shell may be filled with a gas which is a poorer conductor ofheat than air, such as for example carbon tetrachloride or chloroform.

BRIEF DESCRIPTION OF THE DRAWINGS

With the aim of better explaining the invention, a description will nowbe given, by way of illustration and without any limiting nature, ofseveral embodiments thereof represented on the attached drawing, inwhich:

FIG. 1 is a view in axial section of a first embodiment of the jointbetween two pipe sections according to the invention,

FIG. 2 is a view in axial section of a second embodiment of the jointbetween two pipe sections according to the invention,

FIG. 3 is a view of the part III in FIG. 2 on a larger scale,

FIG. 4 is an enlarged sectional view of one variant of the bearing ofthe end of the sleeve in FIG. 2,

FIG. 5 is a sectional view of one variant of the embodiment in FIG. 2,

FIG. 6 is a sectional view representing a device which makes it possibleto create the vacuum between the two shells of a section, and

FIG. 7 is a view in longitudinal section of one variant embodiment ofthe joint in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the connection of the ends of the two pipe sections eachcomprising an inner tubular shell 1 and an outer tubular shell 2.

In this embodiment, near the end of each of the elements of one of theinner shells 1, an overthickness 4 is made by adding metal, the outersurface of which overthickness is then machined so as to make itcylindrically coaxial with the inner shell 1.

A cylindrical collar 6 is fixed at one of its ends by a weld bead 5 onthe overthickness 4 while a shaped piece 7 attached by welding to theend of each of the outer shells 2 is connected onto the circular collar6 by a weld bead 8.

The piece 7 has on its outer periphery a thread 9b whose turns have inthe present case a rectangular cross section.

A cylindrical sleeve 9 comprises at each of its ends a female threadwhose turns 9a also have a rectangular cross-sectional shape.

As can be seen in FIG. 1, there is a large clearance between the turns9a and 9b, so that the sleeve 9, which has been engaged on one of thetwo pipe sections before joining the inner shells 1 by the weld 3, canbe screwed at these two ends onto the turns 9b of the end elements 7 ofthe outer tubular shells 2.

This clearance between the turns 9a and 9b must be sufficient tocompensate on the one hand for the alignment defects of the two innershells 1 joined by the weld bead 3 (parallax defect between the axes ofthe two shells 1 and parallelism defect of these two axes) as well asfor the difference in pitch which may exist between the turns 9b of oneof the tube sections and the turns 9b of the other section.

According to this first embodiment of the invention, a material such asfor example an epoxy resin or an araldite resin, is injected between theturns 9a and 9b, preferably a resin with rapid polymerization and whichpolymerizes without contraction and if possible with expansion.

The manner in which this material occupies the entire space containedbetween the turns 9a and 9b is represented as 10 in FIG. 1.

This filler material can be injected easily between the turns 9a and 9bfrom one or more orifices (not shown) made in the outer wall of thesleeve 9.

FIGS. 2 and 3 represent a second embodiment of the invention.

The inner 1 and outer 2 tubular shells are again seen in FIG. 2.

The inner shells 1 are extended by elements 4a provided with anoverthickness 4 which are welded at 4b onto the inner shells 1.

As in the embodiment previously described, the ends of the inner 1 andouter 2 shells of each pipe section are joined together via collars 6welded at 5 onto the overthickness 4 and at 8 onto the end 7 of theouter shell 2.

The two ends 7 of the outer shells 2 may either be attached by weldingonto the tubes constituting the shells 2 or forged and machined at theends of these tubes.

In order to produce the pipe represented in FIG. 1, steel is first addedon in order to form an overthickness 4 near the ends of the tubesconstituting the shells 1, then the outer surface of theseoverthicknesses is machined in order to make them concentric with theinner shell 1.

The ends 7 of the outer shells 2 are attached by welding onto the tubesconstituting these shells 2, then one end of the collars 6 is welded at8 onto the inner surface of the ends 7 of the outer shells 2.

The inner shell 1, possibly fitted with its insulating coating (notshown in FIG. 1) is then engaged inside the outer shell 2, fitted ateach of its ends with a collar 6. Then, at one end of the tube section,the free end of the collar 3 is welded at 5 to the overthickness 4 ofthe inner shell 1.

According to one preferred embodiment of the invention, at the other endof the tube section, the welding 5 of the collar 6 onto theoverthickness 4 of the inner shell 1 is performed while placing thisinner shell 1 in a state of extension, whereas the outer shell 2 isplaced in a state of compression.

By joining the inner and outer shells of the same pipe section bywelding while the inner shell is stretched and the outer shell iscompressed, a better distribution of the general stresses on the pipewhen a hot fluid is made to flow inside the latter is obtained, whichhas the effect of causing extension of the inner shell 1.

When the various pipeline sections have been produced in this manner,after having engaged a sleeve 9 on the outer shell of each section, thevarious sections are butt joined by the weld beads 3 which thus ensurecontinuity of the inner shells 1.

It is then sufficient to move and screw the sleeve 9 onto the turns 9bof the two pipe sections and to inject the joining material between theturns 9a and 9b in order to obtain the pipe according to the invention.

According to the invention, in order to reduce the number of joints onthe pipe, it is advantageous to produce sections by butt welding atleast two 12 meter tubes to produce the tubular shells of each pipesection.

In this second embodiment of the invention, the sleeve 9 includes at oneof its ends a female thread 9a which is engaged in a corresponding malethread 9b of the extension of the outer shell 2.

At its other end, the sleeve 9 bears against a shoulder 7a by its endwhich does not include any thread.

The part III in FIG. 2 has been represented on a larger scale in FIG. 3.

As can be seen in FIG. 3, the turns 9a and 9b have a bearing flankperpendicular to the axis of the pipe and there is moreover a relativelylarge clearance between the diameters of the turns 9a and 9b.

In this manner, it is possible, according to the invention, tocompensate for errors of parallax which may result from the joining ofthe inner shells 1 by the weld 3.

FIG. 4 represents one variant of the stop of the right-hand end of thesleeve 9 in FIG. 2.

In this variant, there is located between the end of the sleeve 9 andthe outer shell 2 a mobile ring 7b which has on its left a planar faceagainst which the right-hand end of the sleeve 9 can slide and which hason its right a spherical surface which bears against a concave surfaceof the same shape which is produced on the piece 7.

In this manner, the defects in parallelism and in alignment of the axesof the two inner shells 1 of the two sections are compensated for.

In order to carry out the mounting of the pipe according to this secondembodiment, it is sufficient to engage the thread 9a of the sleeve 9onto the thread 9b of the outer shell of the pipe section which issituated on the left, in order that the right-hand end of the sleevebears against the stop 7a, the sleeve 9 then being compressed by virtueof the turns 9a and 9b. The magnitude of this compression of the sleeve9 is easily controlled by the angular rotation which is imparted to itwith respect to the pipe.

According to the invention, the compression communicated to the sleeve 7must be sufficient for the sleeve to remain compressed, whatever thestresses and in particular the bending stresses to which the pipe issubjected when it is fitted or when it is used.

The sleeve 9 is then immobilized by the welds 20 and 21.

FIG. 5 represents a third embodiment of the invention, in which theinner tubular elements 1 do not include a shaped piece but are joineddirectly by a weld bead 5.

In this embodiment, the outer tubular elements 2 are joined by weldbeads 17 and 18, one to a shaped piece 16 fitted with an external thread9b, the other to a shaped piece 15 acting as a stop.

The ends of the elements of the tubular shell 2 may also be forged andmachined so as to take on a corresponding shape.

In this third embodiment of the invention, the collars 6 have anS-shaped cross-section, which allows them to be attached directly by aweld bead 5 to the outer surface of the inner shell 1 and by a weld bead8 to the inner surface of the outer tubular shell 2.

According to a preferred implementation of the invention, the inner andouter elements are respectively set in tension and compressed beforejoining all the ends of the inner and outer shells.

In the embodiment shown in FIG. 5, the sleeve 9 is not welded onto theouter shells 1 and 2, but the space contained between the sleeve 9 andthe inner shells 1 is filled with a substance 19 which may for examplebe a bituminous substance or alternatively a substance in the form of afoam which, in addition to the fact that it occupies the space situatedbelow the sleeve 9 and thus prevents corrosion, supplements the thermalinsulation.

In the embodiment shown in FIG. 5, a plurality of thin sheets 22 whosereflecting power constitutes efficient heat insulation has been placedbetween the tubular shells 1 and 2 of the various sections.

These thin sheets, which are for example, sheets of aluminium or sheetsmade of a synthetic material such as the one known under the trademarkKevlar, are advantageously separated from each other by a cavitystructure 23 made from a material which is a poor conductor of heat.

Spacer members 24 are placed periodically between the inner and outershells so as to maintain the distance between these two shells in spiteof the stresses to which the pipe is subjected, in particular bendingstresses.

These elements, members 24, are advantageously made of plastic and theyhave a shape such that they come into contact with the walls of theshells 1 and 2 only periodically, in order to limit the thermalconductivity.

FIG. 6 represents one embodiment in which a relatively strong vacuum isformed between the inner and outer shells of each section.

For this purpose, an orifice 25 connected to a tubing 26 is made forexample in the overthickness of the piece 4a so as to make it possibleto connect the space contained between the inner and outer shells with avacuum pump.

When the desired vacuum is obtained, it is then sufficient to fill thetubing 26 in order to maintain the vacuum inside the volume in question.

In one variant, the volume between the inner and outer shells may beoccupied by a gas which is a poor conductor of heat, such as for examplecarbon tetrachloride or chloroform.

FIG. 7 represents one variant embodiment of FIG. 1 in which the ends 7of the outer tubular shells 2 each include male turns 9b on which thefemale turns 9a of the sleeve 9 engage with a sufficient clearance tocompensate for the inaccuracies in pitch and the alignment defectsresulting from the welding 3 of the inner tubular elements 1.

In this variant, the inner tubular shells 1 have, in line with thesleeve 9, a zone of sufficient length in which they are independent fromthe collars 6 and from the outer shells 2, so as to be able to undergo,in this zone, sufficient elastic bending for the male turns 9b of theends 7 of the outer shells 2 to be able to bear on the female turns 9aof the sleeve 9, while compensating for the clearance existing betweenthese turns so as to transmit the compression or traction forces betweenthe sleeve and the ends of the outer tubular elements 2, in order toensure continuity of the mechanical strength of the pipe at the joint.

According to the variant represented in FIG. 7, the collars 6, whichhave a central cylindrical part extended by two frustoconical surfaces6a and 6b are welded at one of their ends at 8 to the end of the tubularshells 2 and at their other end at a point 5 which is relatively distantfrom the weld 3 which joins the two inner tubular elements 1.

By way of example, the welds 5 of the two collars 6 onto the innershells 1 may be approximately 1 meter apart.

Such an arrangement can also be used in the embodiment shown in FIG. 1,which has the advantage of ensuring security for the mechanical holdingof the join in the event that the polymerizable substance which isinjected between the sleeve 9 and the ends 7 of the outer shells 2 doesnot occupy the entire volume left free by the clearance between theturns, and thereby does not by itself ensure mechanical solidity of thewhole. In this case, this substance would be partially crushed and theforces would be taken up as has just been described for FIG. 7.

Conversely, it is advantageous in the variant represented in FIG. 7 toinject a substance at the ends of the sleeve, through orifices (notshown), for example a polymerizable substance, which blocks theclearance between the turns, so as to prevent liquids, and in particularwater, from being able to penetrate into the volume contained betweenthe sleeve 9, the collars 6 and the ends of the inner shells 1, whichvolume is advantageously lined with a heat insulating substance.

It is clear that the embodiments which have been described hereinaboveare in no way limiting, and that any desirable modifications can be madeto them.

We claim:
 1. A pipe for carrying fluids, particularly hydrocarbons,having steel tube sections, each comprising an outer tubular steel shellwith at least one of said outer shells having thread thereon, and aninner cylindrical steel shell of smaller diameter, an outer surface ofsaid inner shell being welded in a leaktight manner to said outer shellat each end of said tube sections, while leaving between the two shellsa closed annular volume, wherein the ends of the inner shells of twosuccessive tube sections are welded to one another at each joint betweensaid tube sections, and a sleeve joining the ends of the outer shells isscrewed onto said at least one of the outer shells by a cylindricalthread having at least one face perpendicular to an axis of said sleeve,which includes between respective male and female turns of said threadon said outer shell and said sleeve a sufficient clearance to compensatefor misalignment of the two sections at said joint, and wherein suchthat when traction, compression or bending forces across said joint areexerted on the pipe, said sleeve communicates these forces to the outershells while imparting to the pipe a mechanical strength at said jointwhich is at least equal to that of said tube sections.
 2. A pipeaccording to claim 1, wherein said turns of said cylindrical thread havestraight fronts and said clearance between respective male and femalethread turns is filled with a substance, transmitting the compression ortraction forces expected to be exerted between the sleeve and the outershells.
 3. A pipe according to claim 2, characterized in that collarsare welded to the ends of the outer shells at points away from the endsof the inner shells.
 4. A pipe according to claim 2, characterized inthat the substance transmitting the forces is injected between the turnsof the sleeve and those of the outer shells.
 5. A pipe according toclaim 2, wherein said substance between the turns is a resin thatpolymerizes without contraction or with expansion.
 6. A pipe accordingto claim 2, wherein said substance between the is one of a polymerizableelastomer and a coal pitch which is applied hot.
 7. A pipe according toclaim 2, wherein said resin is an epoxy resin.
 8. A pipe according toclaim 1, wherein said sleeve is fitted at one of its ends with saidinternal thread part which is screwed onto said external thread part ofa first tube section, and an opposite end of said sleeve is appliedagainst a stop integral with the outer tubular shell of a secondneighboring tube section with a sufficient force for the pipe to have,at the joints between the successive sections, a mechanical strength atleast equal to that of the other parts of the pipe.
 9. A pipe accordingto claim 8, wherein said internal thread part and said external threadpart have respective bearing fronts that are substantially perpendicularto the axis of the pipe.
 10. A pipe according to claim 8, wherein saidopposite end of the sleeve presses on a ring which has a planar surfaceadjacent said sleeve, and said opposite end has a convex sphericalsurface which bears freely on a concave spherical surface made on theend of the outer shell.
 11. A pipe according to claim 1, characterizedin that an insulating material is placed in the volume delimited by thesleeve and the inner shells.
 12. A pipe according to claim 1, whereinsaid inner shell and the outer shell of tube section are welded togetherat each end of said tube section, using a collar after the inner shellhas been given an extension with respect to the outer shell whichcorresponds substantially to that which exists when the inner shell issubjected, with respect to the outer shell, to a temperature differencewhich will exist when the pipe is in use and which will cause heating ofits inner shell.
 13. A pipe according to claim 12, characterized inthat, in order to assemble the inner shell and the outer shell of a tubesection, the inner shell is preheated to approximately half thetemperature difference which is to exist between the inner shell and theouter shell when the pipe is used.
 14. A pipe according to claim 1,wherein said closed annular volume between the two shells of a pipesection comprise a plurality of thin sheets of an anti-thermal radiationinsulator said sheets having a thickness of a few hundredths of amillimeter and being made of one of aluminum and Kevlar®.
 15. A pipeaccording to claim 14, wherein said sheets are separated by cavitystructures which are poor conductors of heat.
 16. A pipe according toclaim 1, characterized in that spacer members made of a material whichis a poor conductor of heat are periodically located in the closedvolume contained between the inner shell and the outer shell.
 17. A pipeaccording to claim 1, characterized in that a vacuum is created in thevolume contained between the inner shell and the outer shell.
 18. A pipeaccording to claim 1, wherein a gas which is a poor conductor of heat,is introduced into said closed annular volume between the inner shelland the outer shell.
 19. A pipe according to claim 18 wherein said gasis one of carbon tetrachloride and chloroform.
 20. A pipe according toclaim 1, characterized in that the inner shell and the collars are madeof a metal which has a low coefficient of expansion.